Offshore oil production and storage facilities are typically linked to onshore facilities by at least one pipeline which has been laid along the bottom of a body of water. Large diameter underwater pipelines may also be required to provide a fluid connection between locations on opposite sides of a body of water, for example, a river, or between two points within an open body of water, such as an ocean.
There are many methods and apparatus available for laying a pipeline underwater. Once laid, the pipeline in many instances can remain in an exposed condition on the water bottom. In other instances, however, for example when the water body has significant boat traffic or other human activity, or when the pipeline may be or is subjected to strong currents or other disruptive underwater forces, it is desirable, if not necessary, to protect the pipeline by burying or entrenching it beneath the body of water. Several methods for accomplishing this are well known in the art.
This invention refers to that method in which a burying sled having a water jet cutting means is pulled or towed along the pipeline. As used herein, the term "burying" is meant to refer both to trenching beneath the pipeline, and to trenching beneath the pipeline and then filling in the resulting pipeline filled trench. The sled and in particular the cutting means straddle the pipeline and cut a trench beneath the pipeline as the burying sled is towed forward. As the trench is cut, the pipeline falls into it and is thus safely situated at a level below the water bottom. The trench may thereafter be filled in. Consequently, the pipeline, safe in its protected environment beneath the water bottom, is relatively unaffected by either water traffic or water forces which might otherwise have had an adverse effect upon it.
Burying sleds incorporating water jetting and cutting nozzles extending from the bottom thereof and into the bottom of a body of water, while well known in the art, continue to pose several difficult problems in practical use. The burying sled may typically weigh, in air, 50 tons or more. It also has large physical dimensions and tends to be difficult to maneuver. Consequently, because of its great weight and physical size, great care must be exercised to prevent inadvertent damage to the sled, the pipeline or both. It is thus a very time-consuming expensive, and precise procedure to lower the sled into its operating position, straddling the pipeline.
Typically, the sled is lowered from a barge over a previously laid pipeline as nearly as possible to a position just above the pipeline. A diver helps control the lowering of the sled from the water bottom by signalling to the barge when to stop lowering the sled. Then, the diver, using his own physical effort, may maneuver the sled assembly to properly position the sled in precise alignment to and over the pipeline, with the jetting and cutting nozzles, the claw elements, in position to straddle the pipeline. Once the correct sled/pipeline orientation and alignment are achieved, the diver orders that the sled be lowered the final few feet to the sea floor. Thereafter the diver returns to the surface and entrenching begins.
If the sled must be raised from the water bottom, for example due either to periodic inspection and maintenance of the cutting assembly or to heavy weather at which time the barge must leave the area, significant time will be spent in repositioning the sled assembly over the pipeline. In addition, every time the sled is lowered toward and over the pipeline, the pipeline and its protective anticorrosive coating are vulnerable to damage from impact with the jetting nozzles or claws. Also, the nozzles or claws may be damaged, and if damaged the entrenching operation must be suspended until repairs are made.
Another problem associated with repositioning the sled over the pipeline is the decreasing capacity of the diver to handle and maneuver the sled as the water depth increases. This is because of impairment of the diver's physical capacity and the increasingly shorter durations for which he can remain under water at greater depths. In addition, poor visibility and rough environmental and meteoceanic conditions limit his capability at any depth. The result is either (a) to reduce the amount of time the diver stays down, (b) to prevent the diver from going down to or staying on the water bottom, or (c) to slow the diver's activities thereby increasing the time required for each step of the positioning process. These conditions, in addition to delaying the sled repositioning operation, also pose some additional hazards for the diver.
It is therefore a primary object of the invention to provide an improved method and apparatus for reliably entrenching pipelines using a burying sled having jetting or cutting nozzles. Other objects of the invention are an apparatus and method to safely position the claws or cutting nozzles relative to the pipeline even when visibility is poor, to reduce the chance of damage to either the sled or pipeline, to reduce "down time", to reduce the time required to position the burying sled, and to reduce the number and time duration of diver assisted operations.
Further objects of the invention include a method and apparatus which results in increased productivity, a simpler method of positioning the claws over the pipeline, and reduced vulnerability of damage to the pipeline during the positioning mode.